Four agents, Bru Pel (Brucella abortus cell wall), glucan, pyran copolymer, and thymosin, increased the life span and cure rate of animals bearing leukemia or an alveolar lung carcinoma when these agents were used in concert with chemotherapy. Bru Pel, glucan and pyran copolymer were found to stimulate macrophages to exert a tumoricidal effect. These agents also potentiated tumor cell vaccines. Bru Pel and pyran copolymer treatment alone resulted in retardation of lung tumor growth. Histopathological studies show the presence of reactive macrophages at the lung tumor site. Two pyran copolymer analogs appear ready for a Phase I clinical trial. A pyran copolymer-methotrexate bound complex exerts both an immunostimulatory and antitumor activity. Several chemicals and biologicals shown to activate macrophages also induce cellular interferon synthesis. Interferon appears to be a common mechanism through which macrophages are activated to exert tumoricidal activity. Interferon activated macrophages can be reversibly inhibited by prostaglandins E1 and E2, possibly through the cyclic AMP mechanism. Portal lung irradiation combined with systemic chemotherapy of alveolar lung carcinoma results in an enhanced survival rate.